A molding tool for punching a hole in a workpiece comprises a punch for pressing the workpiece and a die having a cavity for receiving the punch. The die is positioned under the punch. Normally, the workpiece is placed on the die. Then, the punch is displaced downwardly toward the die. A distal end of the punch is inserted into the cavity of the die. As a result, the workpiece is pierced by the punch.
Constitutive materials adopted for the die or the punch include, for example, SK material, SKD material, or SKH material defined by Japan Industrial Standard (so-called high speed tool steel) containing high carbon steel as a major component, super alloy material such as nickel-based alloy and cobalt-based alloy, and superhard material as composite material of ceramics and metal. In order to improve abrasion resistance, the surface of the die or the punch is sometimes coated with a coating film of hard ceramics such as TiC or TiN.
The high speed tool steel and the super alloy material have high strength and high toughness. However, the high speed tool steel and the super alloy do not have sufficient abrasion resistance, compressive strength, and rigidity. The superhard material has high abrasion resistance, compressive strength, and rigidity. However, the superhard material does not have sufficient toughness and tends to cause cracks and breakage. That is, the characteristics of the high speed tool steel and the super alloy material are opposite to the characteristics of the superhard material. Therefore, the constitutive material for the die or the punch is selected in consideration of the constitutive material of a workpiece.
It is desirable that the die or the punch has high hardness, high strength, and high toughness. High hardness, i.e., high abrasion resistance is essential for a long service life. High strength helps to prevent deformation of the die or the punch, even if high stress is exerted thereon in piercing a hole in a workpiece. Further, the die or the punch having high toughness scarcely suffers from the occurrence of cracks and breakage. However, conventional dies or punches do not have all of the characteristics described above.
For example, in the case of the die or the punch composed of a superhard material, it is possible to improve toughness by increasing the composition ratio of metal. However, the superhard material having high metal composition ratio does not have high hardness and strength. Therefore, the service life of the die or the punch may not be long. In contrast, it is possible to improve hardness and strength by decreasing the composition ratio of metal at the sacrifice of toughness. However, the cracks and breakage tend to occur more frequently.
As described above, the superhard material having high hardness and strength does not have high toughness. The superhard material having high toughness does not have high hardness and strength. Therefore, it is difficult to improve all of the characteristics (hardness, strength, and toughness) of the die or the punch.